1. Field of the Invention
The present invention relates to a process for producing polyurethane, and more particularly, to a process for producing polyurethane suitable for molding by reaction injection molding process (abbreviated as RIM process hereinafter).
2. Description of the Prior Art
RIM process has been in practical use for the production of polyurethane moldings to be applied to automotive exterior and interior members such as bumpers, instrument panels, and steering wheels. It is known that the RIM process employs a polyol component which is a polyether polyol having a molecular weight of 1,500 to 12,000 and a chain extender which is a mixture composed of 5-45 wt % of aromatic diamine and 55-95 wt % of low-molecular weight diol. (See Japanese Patent Laid-open No. 74325/1982.)
The conventional polyurethane production process employs a gas loading device to ensure the mold filling of the reaction mix. On the other hand, there is a demand in the polyurethane industry that polyurethane with good moldability be produced by using the existing metering pump of axial plunger type. Unfortunately, the metering pump of axial plunger type has such a small delivery that it cannot be run with a large amount of gas loading required for the complete mold filling.
An aromatic diamine as the chain extender has a disadvantage that it, when used in excess of a certain amount, prevents the molding from electrostatic coating (which is a common finishing process). Thus there is a strong demand for a process for producing polyurethane which is superior in moldability and physical properties and receptive of electrostatic coating.
It is an object of the present invention to provide a process for producing polyurethane moldings with good moldability by using a metering pump which has such a low delivery that it does not permit a large amount of gas loading.
It is another object of the present invention to provide a process for producing polyurethane, said process permitting reducing the frequency of the application of external mold release agents.
It is further another object of the present invention to provide a process for producing polyurethane which is receptive of efficient electrostatic coating.
Other and further objects, features, and advantages of the present invention will appear more fully from the following description and claims.